Numerous compounds have been utilized in vivo and/or in vitro to inhibit ectopic calcification, and bone mineralization and resorption. Diphosphonates (e.g. EHDP, C12MDP) have received considerable research and clinical attention because of their superior ability to inhibit mineralization. However, EHDP may be contraindicated clinically because it produces rickets. Phosphocitrate, a compound chemically indistinguishable from an extremely potent, naturally-occurring inhibitor of mineralization, has been recently synthesized in pure form. It is an even more potent inhibitor of mineralization than EHDP in vitro and preliminary experiments indicate that phosphocitrate is equally as effective as EHDP in preventing soft tissue calicification in vivo. Initial histological examinations of bone suggest that phosphocitrate has little effect upon hard tissues, but a much more comprehensive study of the potential consequences of this compound on bone and cartilage structure and cell function is needed. This project is designed to provide that information. Despite numerous studies, the cellular effects of the diphosphonates are unclear. The proposed comparative investigation should elucidate the mechanisms of action of these compounds. Rat skeletal samples will be examined for structural changes using morphometric analysis techniques at the light and electron microscopic levels. The effects of phosphocitrate and diphosphonates on bone cell (osteoclast, osteoblast and osteocyte) enzyme activities (lysosomal, Golgi apparatus and cell membrane) will be compared using correlative histochemical and biochemical techniques. The differential effects of these compounds on an animal model of soft tissue calcification will also be investigated both histologically and histochemically. Particular attention will be given to differences in the effects of phosphocitrate and the diphosphonates brought about by animal aging in light of the fact that virtually all the previous work in this area has been confined to young animals. The identification of selective inhibitors of normal and ectopic calcification inhibitors can have obvious therapeutic value for both osteolytic and soft tissue disorders (nephrocalcinosis, myositis ossificans progressiva and dermatomyositis).